Can't change display modes

I have written a display driver (first time), and I'm trying to change the
screen resolution. I'm using EnumDisplaySettings to get the modes, and
setting a mode via the ChangeDisplaySettingsEx call. The EnumDisplaySettings
return my various defined modes (I see my DrvGetModes function getting
called), but when I call ChangeDisplaySettingsEx it returns a -1
(DISP_CHANGE_FAILED). I do a CDS_TEST to see if I can set the mode first
before I actually set it. But no matter what flag I put on the
ChangeDisplaySettingsEx call I get the DISP_CHANGE_FAILED failure.



What should I be looking for in my display driver, i.e. what callback
function should I have?

Thanks,
Rick